Histologic, nick end labeling for apoptosis and electron microscopic studies were made of the heart, kidney and small intestine in spontaneously hypertensive rats (SHR) treated for 12 weeks with doxorubicin (1 mg/kg/week), mitoxantrone (0.5 or 0.25 mg/kg/week) or saline (controls). Semiquantitative scoring showed that the severity of the cardiac lesions produced by doxorubicin was comparable to that resulting from 0.5 mg/kg mitoxantrone, but greater than that induced by 0.25 mg/kg mitoxantrone (to which it is therapeutically equivalent). The nephropathy and the intestinal toxicity produced by doxorubicin also were more severe than those resulting from either dose of mitoxantrone. Apoptosis of cardiac myocytes was not induced by either drug, but involved cardiac dendritic cells in SHR given doxorubicin. Apoptosis in renal tubular epithelium was comparable in SHR given doxorubicin and the higher dose of mitoxantrone. Doxorubicin induced more frequent apoptosis in intestinal epithelium than did the higher dose of mitoxantrone. We also show that mitoxantrone and iron (III) form a strong 2:1 complex in which the drug may be acting as a tridentate ligand. This complex, like the iron(III)-doxorubicin complex, may be capable of redox cycling and producing reactive oxygen intermediates (ROI) that damage tissue. Decreased formation of ROI by mitoxantrone may account for its reduced cardiotoxicity compared to that of doxorubicin.